Exhaust brake control system

ABSTRACT

A control system for an exhaust brake which operates to impede exhaust flow from a turbocharged combustion engine when the engine throttle is closed. The control system includes means for delaying application the exhaust brake only when the vehicle throttle is closed to allow the rotational speed of the turbo charger to drop thereby reducing wear to seal components.

TECHNICAL FIELD

The present invention relates to a system for controlling operation of avehicle exhaust brake. In particular, the invention relates to thecontrol of an exhaust brake applied to a turbocharged engine.

BACKGROUND OF THE INVENTION

Exhaust brake systems are widely fitted to compression ignition engines(diesel engines) used to power large vehicles such as trucks. Many typesof exhaust brake system are known, all generally comprising a valve inthe exhaust line from the engine which when activated at leastsubstantially blocks the engine exhaust. This creates backpressure,which retards rotation of the engine providing a braking force, which istransmitted to the vehicle wheels through the vehicle drive train. Withsome exhaust brake systems the brake can be set to activateautomatically when the engine throttle is closed (ie the driver liftshis foot from the throttle pedal). In other systems the exhaust brakemay require manual activation by the driver, such as depression of abrake pedal. Throughout the description and claims, the term enginethrottle is used. Although a diesel engine does not include acarburettor, which has an air throttle, the term is used to describe theliner, mechanism, or system that varies the rate at which fuel isdelivered to a diesel engine and thus its power output. Therefore,whenever the term engine throttle is used, herein, it refers to any ofthese devices.

It is now conventional for large diesel engines to be turbocharged. Aturbocharger essentially comprises an exhaust driven turbine wheel,which rotates a compressor wheel. The turbine wheel and compressorwheels are mounted in respective housings on opposite ends of arotatable shaft which passes through a bore in a bearing housing locatedbetween the compressor and turbine housings. The bearing housing housesjournal and thrust bearings and associated lubrication systems andseals.

The shaft is typically sealed with respect to the bore of the bearinghousing at the turbine wheel end by a shaft seal ring (or piston ring)which sits in an annular groove provided in the shaft behind the turbinewheel. The shaft seal prevents oil from the bearing lubrication systemsfrom leaking into the turbine housing which can cause blue smoke and oildrips from the exhaust pipe, as well as preventing exhaust gas pollutionof the bearing housing which can cause overheating and adversely effectbearing life.

Problems can arise with this conventional shaft seal arrangement wherethe turbocharged engine has an engine brake valve located downstream ofthe turbine. As the engine brake is activated the backpressure in theexhaust line, and thus in the turbine wheel housing, rapidly rises andcan reach 4-5 bar (4-5 times atmospheric pressure). As pressure behindthe turbine wheel thus increases the shaft seal ring can be pushedinboard. This movement, together with the high rotational speed of theshaft, can generate excessive frictional heating, which can cause theshaft seal ring to overheat. This in turn can cause the ring to be moresusceptible to movement in the bore and induce a reduction in life ofthe seal. This problem is particularly pronounced during the running inperiod of the engine/turbocharger before the seal ring “beds in”.

SUMMARY OF THE INVENTION

It is an object of the present invention to obviate or mitigate theabove problem.

According to a first aspect of the present invention there is provided acontrol system for an exhaust brake which operates to impede exhaustflow from a turbocharged combustion engine when the engine throttle isclosed, wherein the control system includes means for determining therotational velocity of the turbocharger and is operable to apply theexhaust brake only when the vehicle throttle is closed and therotational speed of the turbo charger is below a threshold value.

According to a second aspect of the present invention there is provideda control system for an exhaust brake system which operates to impedeexhaust flow from a turbocharged combustion engine when the enginethrottle is closed, wherein the control system is operable to delayapplication of the exhaust brake for a period of time following closureof the engine throttle.

By the simple expedient of delaying application of the exhaust brakefollowing activation of the exhaust brake system, the present inventionallows the turbocharger shaft speed to drop significantly before theback pressure in the turbine housing rises to levels sufficient to exertforces capable of moving the turbine end seal arrangement, such as theseal ring referred to above. This greatly reduces wear to sealcomponents.

The invention may be applied to exhaust brake systems in which theexhaust brake is automatically activated, by for instance releasing theengine throttle, or in which manual activation is required by forinstance depressing a pedal. In the latter case there will already besome delay between closure of the throttle and activation of the brakebut the system according to the invention will operate to ensure thatthe delay is sufficient to allow the turbine speed to drop sufficientlyto prevent wear to the seal as mentioned above. The invention operatesto introduce delay between an exhaust brake activation signal or action(such as closing the throttle or depressing a brake pedal) and actualapplication of the exhaust brake (by closing the exhaust brake valve).In other words, introduction of a delay between activation of the brakesystem and closure of the exhaust brake valve.

The delay period may be pre-determined or may be a function of therotational speed of the turbocharger either immediately prior to closureof the throttle, or as monitored following application of the exhaustbrake.

For instance, one embodiment of the present invention provides a controlsystem for an exhaust brake which operates to impede exhaust flow from aturbocharged combustion engine when the engine throttle is closed,wherein the control system includes means for determining the rotationalvelocity of the turbocharger and is operable to activate the exhaustbrake only when the vehicle throttle is closed and the rotational speedof the turbocharger is below a threshold value.

The present invention also provides a method of controlling activationof an engine exhaust brake which operates to impede exhaust gas flowfrom a turbocharged combustion engine upon closure of the enginethrottle, the method comprising introducing a delay between closure ofthe throttle and application of the engine exhaust brake.

For instance in one embodiment of the invention the method comprisesmonitoring the turbocharger rotational speed, or an engine parameterindicative of the turbocharger engine speed, and activating the exhaustbrake only when the throttle is closed and the turbocharger speed hasdropped below a threshold value.

It will be appreciated that when the throttle is “closed”, the fuelsupply is not necessarily stopped completely, rather the fuel supply maydrop to an idle level. References to throttle closure throughout thisspecification are to be interpreted accordingly. For instance, the actof throttle closure in respect of a vehicle will generally compriserelease of the throttle pedal by the vehicle driver.

Other objects and advantages of the present invention will be apparentfrom the following description.

Specific embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

SUMMARY OF THE DRAWINGS

FIG. 1 is a sectional view of part of a turbine wheel/bearing housingassembly;

FIG. 1 a is an enlarged view of the area of FIG. 1 circled by a dashedline.

FIG. 2 is a graph illustrating the rate of pressure rise in the turbinevs. fall in turbine wheel speed as the engine throttle is closed.

DESCRIPTION OF THE PREFERRED EMBODYMENT

FIG. 1 is a schematic cross-section through part of a turbochargershowing a turbine wheel 1 mounted to a turbocharger shaft 2 whichextends through a bearing housing 3 supported on journal bearings 4. Theturbine end of the shaft 2 is sealed with respect to the bearing housing3 by a shaft seal ring 5 which sits in an annular groove 6 defined inthe shaft 2 as shown in FIG. 1 a which is an enlargement of the areacircled by the dashed line in FIG. 1. In this example the bore 7 in thebearing housing is stepped reducing in diameter to define an annularshoulder 8. This assembly is entirely conventional. It will beappreciated that much of the detail of the turbocharger construction isirrelevant to the present invention and thus is not shown.

In a conventional exhaust brake control system, the exhaust brake isapplied immediately and sometimes automatically after the enginethrottle is closed. This results in a rapid rise in pressure in theturbine housing behind the turbine wheel which tends to push the sealring 5 inboard. Axial movement of the seal ring 5 within the annularslot 6, coupled with the high rotational speed of the turbine wheel andshaft, can create excessive frictional heating and rapidly wear the sealleading to early replacement. The present invention greatly reduces thiseffect by the simple expedient of introducing a time delay betweenclosure of the throttle and actuation of the exhaust brake.

Referring to FIG. 2, this shows how the turbine wheel/shaft speed dropswhen the throttle is closed at time=0. In a conventional automaticexhaust brake control system, the exhaust brake will be immediatelyapplied so that the pressure will rapidly rise in the turbine housing asillustrated by line P₁. The effect of delaying the actuation of theexhaust brake in accordance with the present invention (either byintroducing a predetermined time delay or waiting for the turbochargerto drop below a certain speed) is illustrated by the pressure line P₂. Arelatively short time delay TD (typically of the order of 0.1 to 2seconds) is sufficient to allow the turbine speed to drop significantlybefore pressure levels required to exert substantial axial force on theseal ring are reached. There is thus a corresponding reduction infrictional heating of the seal ring as it moves thus greatly reducingseal ring wear.

The invention is not limited to exhaust brake systems in which theexhaust brake is automatically activated when the throttle is closed.For instance, in a system requiring manual activation of the exhaustbrake, by for instance depressing a brake pedal, the delay will beintroduced following that manual activation to ensure that there issufficient delay for the turbocharger speed to drop as required.

As shown in FIG. 3, an engine 20, incorporates a turbocharger 22 havinga compressor 24+turbine 26. Turbine 26 is fed with engine exhaust gasesthrough manifold 28, causing turbine 26 to drive compressor 24 toproduce pressurized air for delivery to engine 20 through intakemanifold 30. An exhaust brake 32, selectively, blocks exhaust flow fromturbine 32 in response to an actuator 34. Actuator 34 receivesappropriate signals from line 36 connected to an exhaust brake controlsystem 38 to cause exhaust flow from turbine 26 to be blocked orimpeded. Control system 38 receives an external signal from line 40 tobegin or terminate blockage. Line 40 receives a manual operator input oran input reflecting engine throttle closure.

The skilled person will be able to implement the present invention in anumber of ways. For instance, in modern microprocessor controlled enginemanagement systems the required time delay between closure of thethrottle and application of the engine brake could readily be programmedinto the engine management system and feed to the engine brake controlsystem 38. The delay may be programmed as a fixed period of timefollowing closure of the throttle or may be related to the engine and/orturbocharger speed immediately prior to closure of the throttle. Forinstance, the engine management system could incorporate a “look-uptable” relating engine speed or turbocharger speed to required exhaustbrake time delay.

The control system may have an internal time delay as described above,or the rotational speed of the turbocharger could be directly monitoredfrom an appropriate sensor 42 providing a signal to the exhaust brakecontrol system via line 44. The exhaust brake 32 is applied only whenthe throttle is closed and the monitored turbocharger speed has droppedto a predetermined value. Similarly, the exhaust brake control system 38could monitor another parameter of the engine, such as exhaust manifoldpressure through a sensor 46 mounted on manifold 28 and connected toexhaust brake control system 38 through line 48. The signal from sensor46 is indicative of the turbocharger speed. In embodiments of theinvention in which the application of the exhaust brake is directlycontrolled from determination of the turbocharger rotational speed it ispossible that on some occasions there my be no time delay betweenclosure if the vehicle throttle and application of the exhaust brake ofthe turbocharger speed is already below the threshold value when thethrottle is closed.

While it may be convenient to incorporate the exhaust brake controlsystem into an otherwise conventional engine management system, it willbe appreciated that a separate control system could be provided whichresponds to closure of the throttle in any of the alternative wayssuggested above.

The exact duration of the delay may vary to suit any particularturbocharger installation but will generally be a compromise between thedesire to limit wear to the seal ring without significantly degradingexhaust brake performance.

The effect of exhaust brake backpressure on shaft seal ring wear isparticularly pronounced during the running-in period of anengine/turbocharger. Accordingly, certain embodiments of the presentinvention may be designed to introduce a delay into the engine brakeactivation for a limited period only of the exhaust/turbocharger lifecycle. For instance, the exhaust brake delay could be programmed tooperate for only a predetermined number of miles travelled (with thepossibility of re-setting this in the event that a turbocharger sealring or entire turbocharger is replaced during the life of theengine/vehicle).

In summary, the essence of the present invention is the concept ofintroducing a delay in the operation of the exhaust brake followingclosure of the throttle and activation of the exhaust brake system(which may be automatic or require manual activation) and the skilledperson will be able to implement the invention in a variety of differentways suited to different applications.

1. (canceled)
 2. (canceled)
 3. (canceled)
 4. (canceled)
 5. (canceled) 6.A control system for an exhaust brake system which operates to impedeexhaust flow from a turbocharged combustion engine when the enginethrottle is closed, wherein the control system is operable to delayapplication of the exhaust brake for a predetermined period of timefollowing closure of the engine throttle.
 7. A control system accordingto claim 6, wherein said period of time is a function of the rotationalspeed of the turbocharger immediately prior to closure of the throttle,and said control system includes means for monitoring the rotationalspeed of the turbocharger either directly or from an engine parameterindicative of the turbocharger speed.
 8. A control system according toany one of claim 7, wherein means are provided for continuallymonitoring the rotational speed of the turbocharger either directly orby monitoring an engine parameter indicative of the turbocharger speed,and said exhaust brake is applied only when the throttle is closed andthe monitored turbocharger speed has dropped below a predeterminedvalue.
 9. A control system according to claim 6, wherein the controlsystem is operable to introduce said delay for a limited period of thelife cycle of the turbocharger.
 10. A control system according to anyone of claim 6, wherein the engine brake is applied automaticallyfollowing closure of the throttle and after expiry of said time period.11. (canceled)